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Ocean array alters view of Atlantic ‘conveyor belt’

PORTLAND, OREGON—Oceanographers have put a stethoscope on the coursing circulatory system of the Atlantic Ocean, and they have found a skittish pulse that’s surprisingly strong in the waters east of Greenland—discoveries that should improve climate models.

The powerful currents known as the Atlantic meridional overturning circulation (AMOC) are an engine in Earth’s climate. The AMOC’s shallower limbs—which include the Gulf Stream—move warm water from the tropics northward, warming Western Europe. In the north, the waters cool and sink, forming deeper limbs that transport the cold water back south—and sequester anthropogenic carbon in the process. This overturning is why the AMOC is sometimes called the Atlantic conveyor belt.

Last week, at the American Geophysical Union’s Ocean Sciences meeting here, scientists presented the first data from an array of instruments moored in the subpolar North Atlantic. The observations reveal unexpected eddies and strong variability in the AMOC currents. They also show that the currents east of Greenland contribute the most to the total AMOC flow. Climate models, on the other hand, have emphasized the currents west of Greenland in the Labrador Sea. “We’re showing the shortcomings of climate models,” says Susan Lozier, a physical oceanographer at Duke University in Durham, North Carolina, who leads the $35 million, seven-nation project known as the Overturning in the Subpolar North Atlantic Program (OSNAP).

Four years ago, researchers began placing OSNAP’s 53 moorings, studded with sensors to measure temperature, salinity, and current flow, in the waters between Labrador and Scotland. The moorings, galvanized steel wires as thick as a pinkie, are anchored to the ocean floor and tugged vertically by submerged floats. Some moorings are short, designed to measure deep currents near the ocean floor, while others rise nearly to the surface. Since 2004, researchers have gathered data from another array, at 26°N, stretching from Florida to Africa. But OSNAP is the first to monitor the circulation farther north, where a critical aspect of the overturning occurs. It’s here in the frigid Nordic Seas that water masses become cold and dense, sinking in streams that snake along the basin bottom, eventually turning southward and reaching the subtropics in about a decade.

It is thought that the formation of this so-called “deep water” helps drive the AMOC, but the first 21 months of data from OSNAP aren’t conclusive. Both of the recorded winters were unusually cold and created similarly large amounts of deep water, but the strength of the AMOC whipsawed wildly between 8 and 25 sverdrups, a unit of flow roughly equivalent to the total flow of all the world’s rivers. However, this variability was on such short timescales—months—that it might not be linked to the deep-water formation at all, Lozier says. “We need more winters.”

In circulation

Arrays monitor circulating currents in the Atlantic Ocean, in which warm shallow waters move north (red), while cold deep waters move south (blue).

Another reason to study the AMOC in the subpolar North Atlantic is that the rugged ocean floor in this region carves the current pathways up into tortuous tributaries, unlike the relatively smooth flows at 26°N. OSNAP’s stationary moorings cannot trace these meandering pathways, so the array is supplemented by drifting floats. Between 2014 and 2017, researchers deployed 135 neutrally buoyant glass tubes, each roughly 2 meters long, at depths between 1800 and 2800 meters near the southern tip of Greenland. About half of the floats have now surfaced and relayed records of their daily positions to satellites passing overhead, says Amy Bower, a physical oceanographer at the Woods Hole Oceanographic Institution in Massachusetts and an OSNAP principal investigator.

Bower and her team were surprised to find that several floats had been caught off the tip of Greenland in kilometer-scale eddies that were previously known to exist only much farther north. These “eggbeaters,” Bower says, may be stirring up and fragmenting the ribbons of deep water that wind around Greenland.

OSNAP’s ability to ground truth earlier assumptions has climate scientists eager to get their hands on the new data, says Steve Yeager, who works on AMOC simulations at the National Center for Atmospheric Research in Boulder, Colorado. “It provides a really critical benchmark for models.”

At the meeting, researchers working with the 21 moorings of the 26°N array also released their latest findings, which include measurements through February 2017. They show that the AMOC has weakened by about 15% compared with its 2004–08 level. Some climate models have raised the specter of a sudden shutdown of the AMOC—the apocalyptic scenario, leading to a frozen Europe, depicted in the 2004 movie The Day After Tomorrow—and the possibility is also supported by evidence from the geological past. But the decline in the AMOC hasn’t persisted long enough yet to be a cause for concern, says David Smeed, a physical oceanographer at the National Oceanography Centre in Southampton, U.K.

The overall trend of the AMOC will become clearer with time. This summer, researchers on the R/V Neil Armstrong will pull up OSNAP moorings and retrieve readings recorded from 2016–18.